As regards dispersants, also known as plasticizers, for mineral particles and in particular for hydraulic binders, such as cement, mortar or concrete, there exists a category of products possessing noteworthy performances in terms of fluidification of the mixture based on hydraulic binder, that is to say that they make possible good fluidification of the mixture with relatively low proportions of dispersant. Due to these good performances, these products make it possible to prepare mixtures of good fluidity with a reduced contribution of water, hence the term employed in the trade, of “high-range water reducers”, to describe them. This category of products is also known as that of the “superplasticizers” and it essentially comprises two types of families of commercial products, depending on their chemical natures. Numerous works, such as the book by V. S. Ramachandran, V. M. Malhotra, C. Jolicoeur and N. Spiratos, entitled Superplasticizers: Properties and Applications in Concrete, published by the Materials Technology Laboratory, CANMET; Natural Resources Canada, copyright Minister of Public Works and Government Services, Canada, 1998, describe these two families of products as being, on the one hand, that of sulphonated polymers and, on the other hand, that of polycarboxylates. The family of the sulphonated polymers includes polynaphthalene sulphonates and polymelamine sulphonates, also known as the condensation products of formaldehyde with naphthalene sulphonate and melamine sulphonate, respectively. There is a tendency these days for these products to be replaced by the second main family of commercial superplasticizers, the polycarboxylates, the performances of which are much superior in the majority of cases.
In most cases, the term “polycarboxylates” refers to “comb” copolymers, in the molecular structure of which it is possible to distinguish:                a main hydrocarbon chain comprising ionizable groups (of the type of those resulting from the polymerization or copolymerization of acrylic and/or methacrylic acid, maleic and/or fumaric acid, and the like) and        uncharged side polymer chains comprising polyethylene oxide blocks, inter alia or exclusively.        
Two main methods exist which make possible the synthesis of these molecules of copolymers comprising side chains or “comb” copolymers.
The first method, which is the most direct for a person skilled in the art, consists of the copolymerization of monomers, for example of acrylic or methacrylic type, of at least two different types: those, acids, comprising ionizable groups which give rise in a basic medium to negative charges on the main chain, and those, to which are attached, via chemical functional groups, such as esters or amides, oxyethylene chains which give rise to side chains. It is this first method which is used in the process of the invention.
The second method, in two stages, consists of the synthesis, first, of the main chain comprising the ionizable groups and of the subsequent attachment, to this main chain, of the side chains via chemical reactions between functional groups of the main chain (such as acid groups) and functional groups associated with the side chains to be attached, such as alcohol groups (esterification with the acids of the main chain), amine groups (amidation with the acids of the main chain) or others.
The preparation of the “comb” copolymers by the first method described above is generally carried out by a radical copolymerization reaction in a predominantly aqueous medium, according to methods well known to a person skilled in the art. In addition to the choice of the monomers to be copolymerized and their relative composition, a very important aspect of these copolymerizations is the control of the average molecular masses and of the distribution of the molecular masses, as these two variables are intimately related to the applicational properties of the products obtained. Thus it is, for example, that U.S. Pat. No. 6,376,581 discloses the method for the production of a cement dispersant composed of polymers of polycarboxylic acid type: a) the weight-average molecular masses of which lie between 10 000 and 500 000 g/mol (with respect to polyethylene glycol standards and according to a method of measurement by gel permeation chromatography or GPC) and b) the difference between the weight-average molecular mass and the molecular mass at the tip of the peak ( Mw−Mp) of which is between 0 and 8000. The difference Mw−Mp of less than or equal to 8000 reflects a degree of narrowness in the distribution of the molecular masses which is described as necessary for the good performances of the dispersants claimed in U.S. Pat. No. 6,376,581. In point of fact, U.S. Pat. No. 6,376,581 discloses polymers which are prepared by a conventional radical polymerization process, the tendency of which is to naturally result in relatively broad molecular mass distributions, this tendency increasing as the predominant molecular masses (or molecular masses at the tip of the distribution of the masses, Mp) desired increase.
Furthermore, it is well known to a person skilled in the art that the control of the molecular masses of the polymers manufactured by conventional radical polymerization methods in an aqueous medium, such as that disclosed in U.S. Pat. No. 6,376,681 and in other patent applications using this type of polymerization, such as, for example, Patent Application PCT No. WO 01/74736 A1 or European Patent Application No. EP 1 136 507 A1, requires the use of chemical agents or systems for controlling the molecular masses, such as chain-transfer agents, the role of which is to limit the value of the average molecular mass of the polymers obtained, so as to manufacture polymers of use in the applications of the dispersion of mineral particles and more particularly cement particles. Application EP 1 136 507 A1, for example, situates the region of preference in terms of average molecular mass for dispersants for cement between 1000 and 30 000 Daltons (or g/mol) for the number-average molecular mass or Mn. The text of this patent application explains that, when the average molecular masses are too high (for example Mn>30 000 g/mol), the performances in terms of fluidification of the dispersing polymers are not optimum. It is also explained therein that, when the average molecular masses are too low, the dispersing or plasticizing power of the polymers is seen to be reduced, just like the property which some of these polymers have of keeping cement-based mixtures fluid for a period of time which ranges from a few fractions of an hour to a few hours. However, in this patent application, the polymerization is carried out with conventional radical initiators and with chain-transfer agents.
The limitation of the molecular masses to average values compatible with the good performances of the polymers as dispersants and in particular as dispersants for cement-based preparations, when the processes or methods for the preparation of these polymers involve conventional radical polymerization in an aqueous medium, is thus generally obtained by the use of chain-transfer agents well known in radical polymerization. Apart from the disadvantage of having to add at least one additional ingredient during the polymerization stages, some of the most effective transfer agents in an aqueous medium have other disadvantages, which can be their difficulty of handling due to their toxic or odorous nature. Still so as to limit the average molecular masses of the polymers obtained in an aqueous medium, other strategies based on the actual nature of the conventional radical polymerization can be employed in combination with (or in substitution for) the use of chain-transfer agents. These strategies, which are not devoid of disadvantages, can, for example, be the use of the highest possible polymerization temperatures (this having a tendency to naturally limit the molecular masses by relative modification of the values of the kinetic termination and chain-propagation constants) but this is often expensive in terms of energy and reduces the safety margins in the event of losses of control of the polymerization (exothermic reaction). The use of significant amounts of radical polymerization initiators is also a means of attempting to limit the molecular masses but it can prove to be costly and can also reduce the safety margins in the event of losses of control of the exothermic polymerization reaction. Other means of limiting the molecular masses can consist in operating in a fairly dilute medium or in halting the polymerization reactions at incomplete conversions of monomers, in order to penalize the formation of high molecular masses at the end of the reaction, or in using processes for the semicontinuous addition of the monomers with fairly lengthy addition times; however, these strategies have the disadvantage of impacting the productivity of the preparation method and, for this reason, of increasing the overall production cost for the polymer dispersants.
Furthermore, the natural tendency of the conventional radical polymerization in a concentrated aqueous medium to give molecular mass distributions which increase in width as the targeted average molecular masses increase is one of the main reasons which force a person skilled in the art to attempt to restrict the average molecular masses by the use of agents for controlling masses and/or of other convergent strategies, such as those mentioned in the preceding paragraph. In other words, the conventional radical polymerization in an aqueous medium does not make it possible to obtain molecules having good dispersing properties with, at the same time, relatively high average molecular masses (typically having Mn values of greater than 25 000) and low values for the width of the distribution of the molecular masses.
It has now been found that it is possible to prepare, in aqueous solution or in aqueous dispersion, polymers of polycarboxylic acid type by controlled radical polymerization without the use of a chain-transfer agent and that a polydispersity index Mw/ Mn advantageously <2 and an Mn of between 10 000 and 50 000 are obtained.
It has also been found that it is possible to prepare, in aqueous solution or dispersion, polymers of polycarboxylic acid type by controlled radical polymerization without the use of a chain-transfer agent, in order to convert at least 60% of the monomers, followed by a conventional radical polymerization, and that a polydispersity index Mw/ Mn<6, advantageously <5, preferably <4, and an Mw of between 10 000 and 500 000 and advantageously between 30 000 and 150 000 are obtained. Advantageously, at the end of the controlled radical polymerization, a polydispersity index Mw/ Mn<2 and an Mn of between 10 000 and 50 000 are obtained.
The polymers obtained are of use as dispersants for mineral particles, such as cement. They are also of use as plasticizing admixtures or in mixture with other polymers for compositions formed of mineral particles and more particularly based on cement, such as, for example, cement grouts, mortars and concretes or as plasticizing admixtures in cosmetic formulations containing charges and/or pigments insolubles in water and cosmetic solvents and oils or also in ceramic compositions. The products of the invention have better properties than those of the prior art. The polymers of the invention are used in aqueous solution or dispersion and advantageously in the form of the solution or dispersion in which they were synthesized or of the diluted materials prepared from this solution or dispersion. They can also be used in the powder form after drying, for example by atomization, the solution or dispersion in which they were synthesized. For the use in cement or as plasticizing admixtures for compositions formed of mineral particles, it is necessary that the least possible amount of unpolymerized monomers remain in the solution. The polymers obtained are also optionally of use as a mixture with other products or other polymers in cosmetic formulations or paint or ceramic compositions.